An article by Schmidt published in 1970 in Archives of Physical Medicine & Rehabilitation, June 1970 pp 321-327 is exemplary of the use of a so-called Jamar dynamometer with adjustable hand spacing and a sealed hydraulic system as a standardized instrument for determining grip strength, and notes that grip strength has been correlated with hand dominance, overall "physical fitness", normal growth, seriousness of upper extremity injuries, and the success of physical rehabilitation programs.
Van Patten (U.S. Pat. No. 3,672,219) discloses a handgrip dynamometer equipped with an electric transducer which is connected to a volt meter. Kroemer (U.S. Pat. No. 3,670,573) shows a digit dynamometer with a strain gauge transducer whose output is applied to a chart recorder.
A paper by Gilbert et al. published in 1983 in American Journal of Physical Medicine, vol 62 No. 3 pp 135-144 discloses the desirability of providing a graphical record of force in response to a single request for maximal voluntary contraction using a force transducer whose electrical output is recorded on a strip chart, for the purpose of determining whether an apparent inability to produce a normal gripping force is faked, or is genuine. The authors concluded that the ratio of average force to peak force is statistically correlated to the subject's sincerity, and suggested that a "minimum standard indicative of a sincere effort" should be that the average force during the final three seconds of a 5 second contraction should be at least 90% of the peak force. However, such a ratio is not believed to discriminate between a sincere and insincere subject with sufficient reliability to serve as the sole basis for determining a claim for workman's compensation.
Ruis et al. (U.S. Pat. No. 4,235,437) discloses a microcomputer having among its inputs electrical signals indicative of user exerted force from respective transducers and analog-to-digital converters. The microcomputer samples the measured values including various force values and uses that information to calculate the required counterforce to constrain the group of muscles being exercised to a particular path/resistance profile. The force information is also stored for possible subsequent (unspecified) use. Brentham (U.S. Pat. No. 4,566,692) shows another complex computerized exercising device which includes a microprocessor and an analog-to-digital converter which may be used in the rehabilitation field, which has an electrical input provided by a pressure transducer with the data being either stored for "observation at a later date" and/or illustrated graphically to the user.
A study of Chaffin et al published in 1980 in Medicine and Science in Sports and Exercise, vol 12 No. 3, pp 205-211 concerns the use of electromyography in connection with force measurements of both a "maximum" and an extended submaximal contraction to estimate the subject's true maximum voluntary contraction. In theory such a procedure would be applicable to the measurement of grip strength, but the accurate collection of EMG data presents considerable technical difficulty.
It is to be noted that none of known prior art discloses a simple and accurate apparatus for automatically converting the forces exerted by a subject into a reliable indication of whether the subject is sincerely exerting a true maximal contraction.